1. Field of the Invention
The present invention relates to apparatus and methods for charging specimens to a potential.
2. Description of the Background Art
It is often desired to charge a non-conducting substrate to a known or uniform electrostatic potential. For example, this is often desired in a charged-particle instrument, such as an electron microscope, ion implanter, or other such instrument. One common application is the charging (or discharging) of a semiconductor wafer, especially with an insulating layer thereon. For example, the insulating layer may be silicon dioxide or another insulating material.
A common method for such charging utilizes electron flooding. The conventional technique for such electron flooding is now discussed in relation to FIGS. 1A and 1B. FIG. 1A is a flow chart depicting a method 100 of conventional flooding. FIG. 1B is a schematic diagram depicting a configuration for the conventional flooding in cross-section.
In the conventional flooding technique, the flood system is configured (102) to generate an electron flood beam 152 with a relatively high landing energy (typically, a few hundred electron volts) such that the electron yield is greater than one (η>1). The substrate 154 is passed (or is positioned) (104) under the electron flood gun 156, while an electrostatic field is simultaneously applied (106) above the substrate 154. The flood gun may include, among other components, a cathode 157, an anode 158, and a lower bias electrode 159. The electrostatic field may be applied, for example, using the lower bias electrode 159 around the flood gun 156 (and perhaps a grid in front of the flood gun). Secondary electrons 160 are emitted (108) from the surface 153 of the substrate 154 with very low energy (typically a few eV). These low energy secondary electrons 160 move under the influence of the electrostatic field, charging (110) the surface 153 until the electrostatic field is essentially neutralized. This results in a surface potential approximately equal to the potential of the lower bias electrode 159 of the flood gun 156. However, the potentials are not exactly equal because of space charge effects. When the surface charge reaches (112) a steady state, a secondary electron current 160 (equal to the incident flood beam current) continues to leave (114) the surface 153 of the substrate 154 and drift across the gap towards the lower bias electrode 158. These secondary electrons create (116) a space charge situation with a self-consistent field that drives the secondary electrons across the gap from the surface 153 to the flood gun bias electrode 158 and causes (118) a voltage depression at the surface of the substrate 154.
The following is a typical example of configuring (102) the system for conventional flooding of a semiconductor wafer with a layer of silicon dioxide on its surface. The configuration (102) may be performed using a controller 151 that is configured to control the various voltages applied in the system, such as the voltages on the electrodes in the flood gun 156 and the voltage applied to the substrate 154. The cathode 157 may be at a potential of negative three hundred volts (−300 V), and the anode 158 of the flood gun may be at the ground potential of zero volts (0 V), such that the electron flood beam has an energy of three hundred electron volts (300 eV). The voltage bias applied to the wafer substrate 154 (the wafer bias) may also be at electrical ground (0 V). Assuming that the potential at the wafer surface is not too far from the wafer bias, this would result in a landing energy of roughly three hundred electron volts (300 eV). If the substrate surface 153 is to be charged to approximately ten volts (10 V), then the lower bias electrode 159 of the flood gun 156 may be set to ten volts (10 V).
Unfortunately, as mentioned above, the conventional flooding typically causes (118) an unwanted voltage depression at the surface of the substrate 154. In a one-dimensional approximation, Child's Law may be used to calculate an approximation of the voltage depression. For example, given a one centimeter (1 cm) gap and an incident beam current density of one hundred microamperes per square centimeter (100 μA/cm2), the space charge will depress the substrate by roughly twelve volts (12 V), according to a calculation using Child's Law.
In addition to the aforementioned voltage depression, variations in current density in the flood beam and at the beam edges will cause variations in substrate surface charge in the conventional flooding technique. Furthermore, there may be additional substrate potential errors due to voltage drops at the flood gun electrode (caused by resistivity and/or work function effects at the electrode). Moreover, there may be further substrate potential errors due to stray electric fields. In practice, these various voltage offsets give rise to charge variations of a few or several volts across the surface of a substrate. This makes it difficult to flood a substrate to a known or uniform potential with accuracy of better than a few volts.